1. Field of the Invention
This invention relates to a valve drive system for an engine in which skewed intake and exhaust valves are driven by intake and exhaust cam shafts.
2. Description of the Related Art
A system having four or more valves for each cylinder with their reciprocal axes skewed relative to each other is known for an engine to provided a semi-spherical combustion chamber to improve combustion efficiency. In the valve drive system used for this type of engine, because the opening/closing directions of the intake and exhaust valves are different for each valve with respect to the direction perpendicular to the axes of the cam shafts, complicated structures are adopted to transmit the rotation force of the cam shafts to each valve (see JP-A-59-29709, for example).
The valve drive system disclosed in the patent publication is arranged in such a way that one cam shaft is supported in the center of a cylinder head for rotation. A cam surface of the cam shaft is formed parallel to the axial direction of the cam shaft, and two rocker arms are disposed for each intake or exhaust valve between the cam surface and the intake or exhaust valve.
A first rocker arm of one of the two rocker arms is supported for rocking movement on a first support shaft mounted parallel to the cam shaft, and has one end engaged with the cam surface and the other end extending toward the intake or exhaust valve. A second rocker arm is supported for rocking movement on a second support shaft mounted in a direction perpendicular to the axis of the intake or exhaust valve, and has an underside of the rocking end in contact with the intake or exhaust valve. The opposite side (top surface) to the intake or exhaust valve at the rocking end is engaged with the other end of the first rocker arm. That is, the valve drive system is arranged in such a way that movement of the cam surface is converted, by two rocker arms, to movement in a direction parallel to the axis of the intake or exhaust valve.
However, the valve drive system described above has problems of high manufacturing costs and large size because the number of the rocker arms used is high. In order to construct one rocker arm for one intake or exhaust valve to reduce the costs and downsize the system, it is contemplated that the system is arranged in such a way that the cam surface is inclined perpendicular to the axial direction of the intake or exhaust valve, thereby forming a three-dimensional cam and is engaged with the second rocker arm in a sliding relationship.
However, in implementation of this system, a problem arises in lubrication of the contact portion between the three-dimensional cam and the rocker arm. The lubrication of the contact portion is achieved by an oil film of lubricating oil formed between the cam surface of the three-dimensional cam and the sliding surface of the rocker arm. It is well known that the oil film is maintained when the foregoing two components are in line contact, but is broken when they are in point contact.
When the three-dimensional cam is manufactured as an industrial product, the contact state between the cam surface and the foregoing sliding surface tends to be in point contact due to manufacturing defects of the cam surface, and breakage of the oil film leads to wear of the sliding portion. Forming a highly accurate three-dimensional cam surface requires very long grinding work hours, resulting in a significant cost increase.
The aforenoted copending parent application discloses and claims an arrangement wherein the transmission of motion between the cam shaft and the valve can be accomplished by a single rocker arm for each valve. An arrangement is incorporated in the rocker arm construction so that it or its contact points can rotate relative to the pivotal axis of either the rocker arm or its point of contact with the valve so as to accommodate surface imperfections and misalignment. This arrangement generally requires an individual rocker arm shaft for each rocker arm due in part to the skewed relationship of the valve stems.
This presents another problem which is solved by the construction described in that copending application, that being the mounting for the individual rocker arm shafts.
When the engine is provided with a pair of twin overhead cam shafts, as shown in that application, these cam shafts are supported for rotation about parallel axes that lie on opposite sides of a plane that contains the axis of the cylinder bore. This gives rise to a problem in how the rocker arm shafts can be mounted in the engine without unduly complicating the engine construction and also to accommodate a skewed relationship of the rocker arm shafts, if such an arrangement is employed.
It is, therefore, a principal object of this invention to provide an improved cylinder head and rocker arm mounting arrangement for an engine having plural valves per cylinder, twin overhead cam shafts and individual rocker arm shafts for supporting rocker arms for transmitting motion from the cam shaft to the actuated valves.
It is a further object of this invention to provide an improved and simplified cylinder head arrangement for an engine of this type.
Furthermore and as is noted in the parent application, in a conventional sports type, i.e., high revolution engines with a small angle between valve axes and a large angle between the intake and the exhaust passage axes, it is difficult to dispose rocker arms around the cylinder center. That is, in a constitution in which a common rocker shaft passes through a rocker shaft hole bored across multiple cylinders, the rocker shaft hole will end up in intersecting the spark plug well. Further, it is impractical to machine a long, small-diameter rocker shaft hole while maintaining a high precision of parallelism between the rocker shaft hole and the camshaft.
On the other hand, when a constitution is employed in which the rocker shaft is disposed outside the camshaft, arrangement of the intake and exhaust passages inevitably becomes disadvantageous.
In view of the above, an objective of yet another embodiment of the present invention is to provide a valve driving mechanism for a multi-cylinder engine which makes it possible to employ an integral type of cylinder head while disposing rocker shafts between intake and exhaust camshafts, and to increase rigidity of supporting the rocker arms by supporting them within a compact arrangement.